Polyamides are synthetic polymers widely used for the manufacture of diverse shaped articles, including moulded and injected parts, which are often proposed for high the electro-, electronic, and automotive industry.
In these fields of use, the moulded polyamide article during its normal useful lifetime is in contact with a heat source which frequently attains and/or which attains for a longer period temperatures largely exceeding 100° C. The heat source may be a heat producing device or a heated device or may be the surrounding environment wherein the moulded article is placed. Examples of heated devices or heat generating devices are engines, or elements thereof, and electronic devices such as semiconductors. For the automotive segment high-temperature-use application are regularly found in so-called under-the-hood or under-the-bonnet applications, herein referred to as high temperature automotive applications. Therefore, the invention in particular relates to polyamide suitable for the manufacture of moulded articles for use in the electro-, electronic, and automotive industry.
Moulded articles for the electro, electronic and automotive industry and moulding compositions based on polyamides generally have to comply with a complex property profile, including, for the compositions as moulded, good dimensional stability, high heat distortion temperature (HDT) and good mechanical properties, such as a high tensile strength, tensile modulus and fatigue. Polyamide materials generally tend to show a decrease in mechanical properties due to thermal degradation of the polymer. This effect is called heat ageing. This effect can occur to an undesirable extent. In particular with polyamides as the thermoplastic polymer, the deteriorating effect of exposure to high temperatures can be very dramatic.
In attempts to improve heat aging characteristics, it has been conventional practice to add heat stabilizers to polyamide compositions. The function of a heat stabilizer is to better retain the properties of the composition upon exposure of the moulded article to elevated temperature. When using a heat stabilizer, the useful lifetime of the moulded material can be extended significantly, depending on the type of material, use conditions and type and amount of heat stabilizer. Examples of heat stabilizers typically used in polyamides are organic stabilizers, like phenolic antioxidants and aromatic amines, and copper, either in the form of a copper salt in combination with potassium iodide or potassium bromide, or in the form of elementary copper, and metal powders, in particular iron powders.
Existing technologies, while leading to improvements of long-term heat aging resistance, are nevertheless insufficient for more demanding applications, involving exposure to higher temperatures; in many applications, retention of mechanical properties after long-term exposure to temperatures as high as 160° C., or even 180-200° C. and higher becomes a basic requisite. The number of specialty applications, requiring compositions with improved heat ageing properties is also increasing.
On the other side, it is common practice to improve impact properties, and more particularly multiaxial impact properties, of polyamide compounds, so that the same possess increase impact strength, display a ductile behaviour under load, and, in the unfortunate event of fracture, cracks and weak points will ideally remain concentrated around the stressed area and not to propagate, through the addition of tougheners or impact modifiers. Nevertheless, because these additives are generally based on alpha-olefins backbones, they may negatively affect thermal resistance and ageing properties.
The aim of the invention is therefore to provide impact resistance reinforced polyamide compositions, which have better heat ageing properties than the known compositions, thereby providing for the possibility to make moulded articles that can be used at higher continuous use temperatures than the moulded articles prepared with the known compositions and which possess outstanding impact strength.
Within this scenario, WO 2007/036929 (NILIT LTD) Apr. 5, 2007 discloses, notably, glass fiber reinforced polyamide compositions, wherein the polyamide is modified by a polyhydric alcohol chemically bonded at least to a part of the polyamide. This document is silent about heat aging properties of said compositions.
Further, US 2010029819 (DU PONT) Feb. 4, 2010 teaches that glass fiber reinforced polyamide compositions comprising one or more polyamide, and one or more polyhydric alcohol (in amount of 0.25 to 15% wt), and optionally a polymeric toughener deliver improved thermal resistance.
Still, WO 2012/140100 (RHODIA OPERATIONS) Oct. 18, 2012 is directed to the use of polyhydric alcohols in polymerization of polyamides, so as to manufacture polyamide modified by incorporation in the polymer chain of said polyhydric alcohols, for achieving thermal stabilization of the polyamide. Hence, it discloses polyamides modified by a polyhydric alcohol chemically bound to the polyamides, which can be formulated with fillers and impact modifiers.
The Applicant has now found that by the incorporation in reinforced compounds based on polyamides modified with polyhydric alcohols of certain amounts of impact modifying rubber and copper-containing stabilizer is effective in delivering outstanding synergetic heat aging stability effect, in particular delivering outstanding retention of mechanical properties even after long term exposure to temperatures as high as 210° C., while simultaneously providing toughening effect.